1. Field of the Invention
The present invention relates to a MOSFET (Metal Oxide Semiconductor FET (Field Effect Transistor)) drive circuit which drives a MOSFET, a programmable power supply and a semiconductor test apparatus which include this MOSFET drive circuit, and more particularly, the present invention relates to a MOSFET drive circuit which can realize high-speed switching of the MOSFET, a programmable power supply and a semiconductor test apparatus.
2. Description of the Related Art
A programmable power supply used in a semiconductor test apparatus includes a function to measure an electric current consumption of a DUT (Device Under Test) by applying a voltage to the DUT (voltage application current measurement function), a function to measure an output voltage of the DUT by flowing a current in the DUT (current supply voltage measurement function) or the like.
Such a programmable power supply can generally supply a current up to approximately 1 A, but a power supply current of a DUT varies depending on each product class and is as very wide as several-μA to 1 A. Therefore, for example, when measuring a power supply current of a DUT by using the voltage application current measurement function, a measurement range in the programmable power supply must be switched in accordance with a standard value of the power supply current of the DUT as a measurement target in order to increase the accuracy of a measurement result.
FIG. 6 shows a circuit configuration of this conventional programmable power supply.
As shown in the drawing, a programmable power supply 10 of a semiconductor test apparatus 1 includes a DA converter 11, resistors R12 and R13, a differential power amplifier 14, a buffer amplifier 15, a small-current measurement portion 16, and a large-current measurement portion 17 as main structures.
Here, the large-current measurement portion 17 is a part which measures a large current in power supply currents of a DUT 30, and has current detection resistors R17-11 to R17-1n, a potential difference detector 17-2, an AD converter 17-3 and a switch portion 20.
Of these members, the resistor R17-11 is a resistor which constantly flows there through a power supply current, and the resistors R17-12 to 17-1n are flow dividing resistors which enlarge a measurement range. These resistors R17-1 to 17-1n can use the same low resistance value Ra.
The switch portion 20 is a part which performs switching so as to supply a current to the flow dividing resistors R17-12 to R17-1n corresponding to a range which is set at the time of current measurement and, on the other hand, so as not to supply a current to the flow dividing resistors R-12 to R17-1n which do not correspond to this range. As shown in FIG. 7, the switch portion 20 has a MOSFET portion 21 and a MOSFET drive circuit 100.
The MOSFET portion 21 has a structure in which two MOSFETs form a pair. The number of the provided MOSFET portions 21 is the same as that of the flow dividing resistors R17-12 to R17-1n, and a flow dividing branch circuit is constituted by a combination of one flow dividing resistor R17 and the MOSFET portion 21 which determines whether a current is caused to flow through this resistor. Further, the plurality of flow dividing branch circuits are provided and connected with each other in parallel, thereby constituting a flow dividing circuit.
The MOSFET drive circuit 100 is a circuit which drives each MOSFET provided in the MOSFET portion 21, and has a light emitting element 110, a photodiode 120 and an optical MOS 130.
The light emitting element 110 emits light beams upon receiving an input signal from an input side.
The photodiode 120 charges a gate of the MOSFET of the MOSFET portion 21 upon receiving the light beams from the light emitting element 110. As a result, a current is caused to flow through the resistor R17 connected to that charged MOSFET, and a current based on a corresponding range can be measured.
The optical MOS 130 discharges the gate of the MOSFET of the MOSFET portion upon receiving the light beams from the light emitting element 110. As a result, a current is prevented from flowing through the resistor R17 connected to that discharged MOSFET.
It is to be noted that electrical insulation is achieved between the light emitting element 110 and the photodiode 120 and between the light emitting element 110 and the optical MOS 130.
Furthermore, FIG. 8 shows a relationship between a signal (input signal) from the input side of the switch portion 20 and a state of the MOSFET (ON state/OFF state).
First, when the input signal is inputted, the photodiode 120 is turned on, and the optical MOSFET 130 is turned off. As a result, the gate of the MOSFET of the MOSFET portion 21 is charged and turned on.
On the other hand, when the input signal is not inputted, the photodiode 120 is turned off, and the optical MOS 130 is turned on. As a result, the gate of the MOSFET of the MOSFET portion 21 is discharged and turned off.
By switching the signal to be inputted to the input side of the switch portion 20 in this manner, ON/OFF of the MOSFET of the MOSFET portion 21 can be changed over. As a result, a current flows through the flow dividing resistors R17-12 to R17-1n connected to the MOSFET which have entered the ON state, and a current can be measured in a range corresponding to these flow dividing resistors R17-12 to R17-1n (see, e.g., patent reference 1: Japanese Patent Application Laid-open No. 11-006860).
However, in the above-described conventional programmable power supply, a time until charging the gate of the MOSFET is completed after the input signal is inputted or a response time until discharging starts after no input signal is inputted requires a considerable time.
For example, although an output current of the photodiode is used to charge the gate of the MOSFET, since this output current is relatively small, a considerable time is required until charging the MOSFET is completed (“t1” in FIG. 8).
On the other hand, although discharging the gate of the MOSFET is performed by using the optical MOS, discharging the gate of the MOSFET cannot be started before the optical MOS is charged, and hence start of discharging of the MOSFET is delayed for a time which is required to charge the optical MOS (“t2” in FIG. 8).
Specifically, the time until charging the gate of the MOSFET is completed after the input signal is inputted (t1) or the response time until discharging starts after no input signal is inputted (t2) is approximately several-ten μs to several ms.
Such a situation means that switching ON/OFF of the MOSFET is not rapidly carried out, and leads to a problem that a large current cannot be rapidly switched in a current range or an output relay in the programmable power supply or the semiconductor test apparatus.
Thus, as a solution to this problem, connecting many photodiodes can be considered.
According to this solution, since the output current is increased, a charge time of the gate of the MOSFET can be assuredly reduced.
However, connecting the photodiodes as many as the charge time can be greatly reduced increases a circuit area. Therefore, there occurs a new problem that this solution cannot contribute to a reduction in size of the programmable power supply or the like.